1. Field of the Invention
This invention relates to thick phase holograms and more particularly to latent-image methods of their production.
2. Background
Thick refractive index pattern recordings (approximately 10 .mu.m to 10 mm thick) are considered a potentially attractive medium for efficient image reconstruction or conversion. A contemplated procedure for fabricating thick phase patterns involves first recording the pattern as a latent image of relatively small changes in refractive index and then developing the recorded image to produce the needed much larger refractive index changes. Such a latent image procedure is particularly desirable for making high efficiency recordings. If a substantial fraction of the image develops during the recording or exposure step, this much larger refractive index modulation can introduce significant scattering, and hence distortion, of the recording beams. Thus, noticeable distortions of the final image can occur.
Various approaches have been taken to produce latent-image thick phase recordings, e.g., exposure of dichromated gelatin or a photographic film followed by processing with respectively isopropyl alcohol or a bleaching agent. (See Meyerhofer, RCA Review, 33, 110 (1972), and Pennington and Harper, Applied Optics 9 (7) 1643 (1970).) Presently, latent-image techniques can, indeed, yield high efficiency recordings. Latent imaging, however, usually entails elaborate chemical diffusion procedures to develop the image. These chemical procedures can be inconvenient and can also degrade image quality. For example, to develop a dichromated gelatin or photographic emulsion, chemical solutions must be diffused into the photosensitive substrate to develop the refractive index pattern. The substrate prevents diffusion through thickness greater than a few tens of microns. This is a limitation for applications where thicker records are desirable. Additionally, the development process often causes a change in thickness of the substrate. This change, which may or may not be uniform, in turn produces a corresponding distortion of the final image.